Feature request: provide gizmo lines for applied external forces

[viccuad]

Hi, this project is great, congrats and thanks!

While trying it, I've realized that iterating through game dev can be painful.

Suggestion

• Provide gizmo lines as part of the PhysicsGizmos that showcase the externally applied forces. I suppose with a simple line in the normal of the force would be enough.
• Provide a configurable option in PhysicsDebugConfig to configure the maximum length of the lines, and the expected maximum force. This way one can tune the scale of the lines for them to be visible and useful.

[Jondolf]

Hi! In #324, I added publicly accessible contact impulses (for forces, divide by delta time) and support for rendering them. The scale can also be customized, although you can't set the maximum length at the moment.

This is only for contacts for now, and doesn't include gravity, ExternalForce, or joints. I could definitely support rendering those too though, and especially joints might be useful.

It might also be nice to visualize the total force or acceleration of a body. This would include gravity, ExternalForce, and all of the contact and joint forces. Things like the user abruptly setting the velocity to a specific value probably shouldn't be counted towards the force though.